The present invention relates to a comparator circuit for analog-to-digital converter, especially to a comparator circuit with equalization effects that can be used in an analog-to-digital converter.
The analog-to-digital converter (ADC or A/D converter) is a useful circuit in the application of signal processing. An ADC is used to convert analog signals into digital signals. Most frequently used ADC""s include flash ADC, interpolative ADC, folding ADC, two stage ADC etc.
FIG. 1 shows the circuit diagram of a conventional flash ADC. This flash ADC was disclosed in U.S. Pat. No. 5,835,048 as a prior art. As shown in this figure, such a flash ADC comprises a thermocode to binary signal decoder 10 that converts input signals into binary codes according to thermocodes generated by a comparator circuit to be described in details hereinafter. The thermocodes are generated as the result of comparing the voltages of the input signals with reference voltages in a series of comparators 24a, 24b, 24c and 24d. The reference voltages in the comparators 24a, 24b, 24c and 24d are in a constant ratio. For example, the reference voltage of comparator 24a may be greater than that of comparator 24b for an LSB (a least significant bit) of the ADC.
In order to compare the voltage of the input signals with the reference voltages, the input signal Vin is input from an input line 20 and is forwarded to all comparators. A reference voltage Vref is input from a reference signal line 20 and is regulated by resistors 18a, 18b, 18c and 18d, such that reference voltages in the respective comparators are in a predetermined ratio. The operation of the comparator circuit is well known to those skilled in the art and detailed description thereof is thus omitted.
In order to enhance the accuracy of the comparator circuit, preamplifiers 14a, 14b, 14c and 14d are provided at the upstream position to comparators 24a, 24b, 24c and 24d, respectively. The function of the preamplifiers 14a, 14b, 14c and 14d is to amplify the input signals and the reference voltages, such that the small differences between the voltages of input signals and the reference voltages may be amplified. In such a circuit, the input signals Vin and the reference voltage Vref are first input to the preamplifiers 14a, 14b, 14c and 14d and are then forwarded to comparators 24a, 24b, 24c and 24d. After the voltage of an input signal is compared with the reference voltages, a thermocode representing the result of the comparison is generated. In this circuit, the offset voltage of the comparators and the preamplifiers may bring a decisive influence to the accuracy of the comparator circuit.
Take the CMOS flash ADC as an example. If the requirement of its resolution is 10 bits, 1,024 comparators will be needed in the comparator circuit. When the input voltage of the circuit is 2V, the offset voltage of every comparator shall be smaller than 1 mV (xc2xd LSB), such that correct results of comparison may be obtained. However, in an ordinary CMOS comparator, its offset voltage can be tens of mV. Even though it is possible to provide preamplifiers at the input of the comparators to amplify the input signals, the reaction speed of the total circuit will be damaged due to the high gain values of the preamplifiers. Although it is also possible to enlarge the space of the transistors at the input of the amplifiers to reduce the offsets of the preamplifier brought by errors existing in the manufacture process, the total space occupied by the circuit will become too large for an ADC and the over loading of capacitance in the preamplifiers will damage the operation speed of the circuit. In addition, the offsets in the preamplifiers themselves will damage the accuracy of other components of the comparator circuit.
In the conventional art, the above-said problems may be solved by using an averaging circuit. FIG. 2 shows an analog-to-digital converter with improved cell mismatch compensation as disclosed in U.S. Pat. No. 5,835,048. In this circuit, a string of resistors are provided to the outputs of the preamplifiers to average the offsets of the preamplifiers. After the averaging, the accuracy of the circuit may be improved, since the offsets of the preamplifiers are averaged.
Another approach to solve the above-said problem is disclosed by U.S. Pat. No. 5,175,550. This invention also used a string of resistors provided at the output of the preamplifiers to average the offsets of the preamplifiers. The major difference between the ""048 patent and the ""550 patent rest in that the ""048 patent used an active load and the ""550 patent used a passive load. As a result, in the ""550 patent the resistors are used as the load of the preamplifiers and in the ""048 patent the transistor with a high output resistance is used as load. The ""048 patent provided an improvement over the ""550 patent, because the smaller the averaging resistance (the resistances connected in the horizontal direction) is, the better the averaging effects, i.e., the smaller the offset voltage, will be. However, when the averaging resistance is reduced, the gains of the preamplifiers will also be reduced. If the averaging effects of the circuit of both patents are the same, i.e., if both have the same averaging resistance, preamplifiers with greater gains will still have greater gains, even if the averaging resistors are added. In other words, when the space used by the resistors is the same, a preamplifier wherein the resistor is used as load will have a smaller gain than that of a preamplifier wherein the current source is used as load. This is the major improvement provided by the ""048 patent.
Although the above-mentioned inventions are capable of averaging the outputs of the preamplifiers, it inherited drawbacks as well. In these conventional arts, when the resistance of the resistors applied to the outputs of the preamplifiers is small, gains of the preamplifiers will not be sufficient to provide their functions. On the other hand, if the resistance of the resistors is raised, the averaging effect of the resistors will be damaged.
Generally speaking, the averaging resistor shall have a resistance of several K Ohms to tens of K Ohms to provide averaging effects. In such a range, the amplifying effect of the preamplifiers may also be maintained. However, the total space occupied by the resistors will become too large to be included in a commercially available IC chip.
In addition to that, applying averaging resistors at the outputs of the preamplifiers does not solve the influence brought to the comparator by the offset of the input voltage.
It is thus a need in the industry to provide a novel comparator circuit for an analog-to-digital converter which may effectively reduce the offset of the comparator circuit.
It is also necessary to provide a novel comparator circuit for an analog-to-digital converter in which the offset voltages of the comparator circuit may be averaged.
It is also necessary to provide a novel comparator circuit for an analog-to-digital converter wherein offsets of signals may be effectively reduced without the need of providing additional space for circuits.
The objective of this invention is to provide a novel comparator circuit for an analog-to-digital converter which may effectively reduce the offset of the comparator circuit.
Another objective of this invention is to provide a novel comparator circuit for an analog-to-digital converter in which the offset voltages of the comparator circuit may be averaged.
Another objective of this invention is to provide a novel comparator circuit for an analog-to-digital converter wherein offsets of signals may be effectively reduced without the need of providing additional space for circuits.
According to the comparator circuit for analog-to-digital converter of this invention, the comparator circuit may be used in an analog-to-digital converter comprising: an input voltage signal line; a reference voltage signal line; a plurality of comparators connected in parallel to said input voltage signal line and said reference voltage signal line; a plurality of amplifiers corresponding separately to each of said plurality of comparators and connected respectively between said input voltage signal lines, said reference voltage signal lines, and their corresponding comparators; and a thermocode channel connected to outputs of said plurality of comparators in parallel; wherein a plurality of resistors with resistances in a constant ratio are provided in said reference voltage signal line and each resistor is positioned between the inputs of two adjacent amplifiers. In said comparator circuit for analog-to-converter, a plurality of averaging capacitors are provided and each averaging capacitor is positioned between the outputs of two adjacent comparators; wherein said plurality of averaging capacitors may have the same capacitance.
These and other objectives and advantages of this invention may be clearly understood from the detailed description be referring to the following drawings.